Values

Is either a non-negative explicit <length> value or a keyword denoting the thickness of the left border. The keyword must be one of the following values:

thin

A thin border

medium

A medium border

thick

A thick border

The specification doesn't define precisely the thickness of each fo the keywords, which is therefore implementation specific. Nevertheless, it requests that the thickness does follow the thin ≤ medium ≤ thick pattern and is constant on a single document.

inherit

Represents the calculated value of the border-left-width of the parent's element.